Icephobic coatings are being investigated as functional coatings for laminar flow. The coatings are intended to support the wing ice protection systems by reducing the formation or adhesion of ice and consequently reducing the energy requirements of the system. Currently, the most comprehensive method for measuring and understanding the icephobic nature of a coating is by performing icing wind tunnel tests. These tests require large dedicated wind tunnel facilities and would be too costly and impractical for production batch quality control and release testing purposes. For this reason a significant part of the research was dedicated to the development of suitable ‘bench top’ test methods that allow accurate measurement of the icephobic nature of a coating, providing an indication of the long term icing performance of the tested surface.
GKN intended to achieve this goal by determining the key contributing attributes to icephobicity of a surface and generating a predictive model, where predictions of performance can be derived from results of readily-available laboratory analytical techniques. This has not been achieved in entirety before and so to mitigate risk GKN also attempted to develop in tandem a bench top ice adhesion test method which provides the same order of results as would be observed in an icing wind tunnel.
Icephobic coatings are intended to support wing ice protection systems by reducing the formation or adhesion of ice and consequently reducing energy requirements of the system (and thus fuel costs). Development of such coatings can be complex and costly, in part due to the expense associated with icing wind tunnel testing – the only method available for characterising anti-ice behaviour of a surface which is representative of the aircraft operational environment.
The goal of this project was to develop a less expensive but suitable test alternative, which could predict anti-ice coating performance in an icing wind tunnel, and thus the operational environment. Two approaches were taken towards achieving this goal. The first approach attempted to identify chemical and physical characteristics of a surface which contribute to ice adhesion resulting from icing in a wind tunnel. This was done by characterising the surface of the coated test plate using a variety of analytical tests, and comparing these results to ice adhesion test results obtained in an icing wind tunnel by icing the test plate and recording the electro-expulsive energy required to remove the ice, in order to determine trends.
The ideal target was a predictive model, whereby, based upon surface characteristics measured in a laboratory, performance of a coating in an icing wind tunnel (energy required to de-ice) could be predicted. This objective was considered a major challenge to meet, and indeed it was only met in part, in that a validated predictive model was not produced. However, characteristics of a surface which may contribute to ice accretion rate and ice adhesion were identified and/or tested. Some of these characteristics have not been found to have been reported elsewhere, and their identification may lead to further research, which in turn may contribute to an increased understanding of ice accretion and adhesion to surfaces.
The second approach towards achieving the goal of a test method alternative to the icing wind tunnel was development of a laboratory-based icing and ice adhesion test device, which provides a good degree of capability to predict anti-ice performance of coatings in an icing wind tunnel, and thus the natural environment. To this end, a laboratory-based, fully-enclosed icing and ice adhesion test device, which has conditions controllable to enable a high level of natural environmental representation, has been designed, developed, manufactured and tested for fitness for function. The equipment has indeed been proven fit for function, and has been found capable of producing environmentally-representative conditions, for example in terms of pressure, humidity, freezing rain droplet size, and ice type, and once the test specimen is iced, a shear ice adhesion test is performed in situ without need to remove the sample or open the test chamber. The tests performed with the equipment on various coated surfaces during the project suggest good correlation with icing wind tunnel test data and thus achievement of the project goal.
Due to complexity in the design and manufacture, and the large number of parameter settings it is possible to vary, some further work is required to fully determine capabilities of the equipment. However, at the point of closure of this project, the test equipment developed shows great promise for use as a method to increase the understanding of ice adhesion properties of surfaces, to reduce cost and duration of programs targeted at development of aircraft surfaces with reduced ice adhesion, and to batch test anti-ice coatings for quality assurance purposes once developments have been productionised.